CollapsableEventDrawer creates issues when another editor uses FoldoutHeaders

[burakomer]

We have been facing this issue with a package named AnimationSequencer which has a Custom Editor for its Inspector. It places UnityEvents inside FoldoutHeaders. We have been getting the error "You can't nest FoldoutHeaders, end it with EndHeaderFoldoutGroup."

CollapsableEventDrawer needs a guard clause that checks existing foldout headers.